Field of the Invention
The present invention generally relates to lithography, and more particularly to a pneumatic bearing.
Background Art
Lithography is widely recognized as a key process in manufacturing integrated circuits (ICs), as well as other devices and structures. A lithographic apparatus is a machine, used during lithography, which applies a pattern onto a substrate, such as onto a target portion of the substrate. During manufacture of ICs with a lithographic apparatus, a patterning device (which is alternatively referred to as a mask or a reticle) generates a circuit pattern to be formed on an individual layer in an IC. This pattern can be transferred onto the target portion (e.g., comprising part of, one or several dies) on the substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (e.g., resist) provided on the substrate. In general, a single substrate contains a network of adjacent target portions that are successively patterned. Manufacturing different layers of the IC often requires imaging different patterns on different layers with different reticles. Therefore, reticles and substrates must be changed during the lithographic process. To facilitate reticle handling, stages that support reticles are provided with pneumatic bearings.
A bearing is a device that reduces friction between moving parts, and/or supports moving loads. There are two main types of bearings. An anti-friction bearing minimizes friction using devices such as roller bearings or ball bearings. The friction bearing minimizes friction using active lubrication or other means to facilitate motion between moving parts. Friction bearings are also known as sliding bearings. Many bearing assemblies take advantage of both principles—e.g., a lubricated ball bearing assembly.
A pneumatic bearing is an example of a friction or sliding bearing. It uses compressed gas to create a consistent gas film upon which a bearing surface rests and moves. The gas film acts as a virtually frictionless lubricant that facilitates smooth motion between the surfaces of the pneumatic bearing. The bearing surface upon which the lubricating gas film is generated is called the “active surface.” Typically, pneumatic bearings require at least a steady source of compressed gas to maintain the lubricating gas film.
As introduced above, an exemplary environment for pneumatic bearings is in the semiconductor lithography field. There, pneumatic bearings provide a number of advantages. Pneumatic bearings are virtually frictionless, and therefore produce no particulate wear materials as they operate. Such particulate matter would be troublesome in the ultra-clean semiconductor manufacturing environment. Additionally, lubricants present in ball or roller bearings could outgas contaminant molecules, which are also detrimental in semiconductor manufacturing environments. Pneumatic bearings also require relatively little maintenance or regular repair.
Despite all of these benefits, incidental “dry” contact between pneumatic bearing surfaces can occur during use, scratching conventional pneumatic bearing surfaces and compromising their performance. To combat this problem, conventional bearings ride on guideways of polished granite or chrome-plated steel, while the bearings themselves are fabricated with case-hardened stainless steel. The case-hardening process provides wear resistance, corrosion resistance, and resistance to galling by hardening a skin layer of the steel. The bulk of the steel remains unhardened, thus the hardness is maximum at the surface, and drops off rapidly and continuously through the thickness of the skin layer as a direct function of distance from the surface. Unfortunately, the case-hardening process also slightly deforms the bearing, requiring the bearing to be reground, which partially and somewhat unevenly removes the hardened layer. The final product is a functional, flat bearing, however the thickness of the hardened skin layer is not uniform across the bearing surface, and also varies from bearing to bearing, depending on the depth of grinding that was required to regain flatness. In a practical sense, the hardness of the bearing cannot be tightly controlled with the case-hardening process.
Non-uniformity of the hardened skin layer is aggravated in the case of differential seal bearings used in lithographic reticle handler modules, which are an order of magnitude larger than stage bearings, and are made from thin steel plates. Thin steel plates tend to permanently deform with any treatment that requires heating the part, including the case-hardening process. In a vacuum environment, case-hardened steel provides good resistance to galling, does not generate excessive amounts of particles, and is relatively inexpensive. Therefore, it has conventionally been the surface treatment of choice.
Improvements in pneumatic bearing design are constantly needed. This is especially true in the semiconductor lithography tool arts, where manufacturing tools are constantly pushed to more precise tolerances and faster speeds.